Internet connection sporadically dies but not LAN connectivity

You can also try to upgrade your D-Link firmware.

I should add that I've never had this problem in the several years I've had the network up, but it started happening as soon as I connected my CNC machine to the router.  The CNC machine is a fairly complex piece that has its own network with a router, hub, and several internal computers connected to the hub.  I have communication between the office network computers and the CNC machine's controller, but then I sporadically started losing communication over the WAN port of the router.  This is why I'm wondering if my little home router is getting overloaded and thus is undersized for the job.

I checked my DSL modem's logs and don't see anything regarding a change of IP address in that timeframe.  The problem was happening at what seemed to be regular intervals.

This morning I decided to unplug the CNC machine from the D-Link router to try to isolate the problem.  Everything worked fine for most of the day today.  No problems accessing the internet, pinging the DSL modem, etc.  This afternoon I plugged the CNC machine back into the router and successfully pinged the router from the CNC machine.  I went back to my computer and was unable to ping the DSL modem or access the internet.  I did a "Repair" on my Local Area Connection in the Network Connections section of the control panel, and all of a sudden I was able to ping the modem and access the internet.  It looks like somehow the addition of the CNC machine is the culprit.  The modem's outside IP address has been the same all day, so it's not a case of the dynamic IP address changing.

My question is: is this being caused by the D-Link router becoming overwhelmed with traffic, or could there be another cause?

I think that you might have some coalision in you network. You said that you have CNC with own network, hub and router, and after that you conects the CNC direct into your other router. Does CNC have two network cards or you disconnect it from CNC network. I think that (as far as I can imagine your network) your CNC network ends with router, and that router should connect to other router (D-link). Of course there might be need to reconfigure some IP's.

Also if you have two routers in your network there should be only one DHCP server...

I'm sure the topology is a bit confusing so I've included a simple diagram.  The CNC's router plugs into a numbered port on the D-Link router.  CAD Station (my personal computer) plugs into another numbered port on the D-Link router.  The CNC network comes pre-configured from the factory.  Essentially I'm trying to get the Controller PC on the CNC network (192.168.100.101) to communicate with CAD Station on the office network (172.16.0.103).

I appreciate the help.

If i were you.

1) Reset the dsl Modem.
2) Reset the router d-link di-524
3) Reset Laser Router Funkwerks bintec R232b
3) Conect dsl-modem in WAN PORT of d-link (This is your only WAN port that you are going to use)

So. in this moment, you can choice. Conect a direct cable from de
Lase hub to a Lan port of d-link, or just use only the "LAN PORT"  of
"Funkwerkers bintec R232b"

The rest of your computer, only to "get an automatic ip"

If you need to separet for ip address, you just need to know the default
gateway of the  dsl-modem, for example.

172.16.0.1
255.255.255.0
200.44.32.12 <--- Example of your dsl router
 and the DNS  of your dsl

and the other group

192.168.100.1
255.255.255.0
200.44.32.12 <--- Example of your dsl router
 and the DNS  of your dsl

I think, in this way, you are not going to have the same issue of internet conecttion.

I found the manual for your bintec, 450 pages...Take a look at page 129 there is explained how to connect two subnets.

I appreciate the comments thus far, but unfortunately I'm still having the same problem.

If I set the IP address of the bintec's ethernet interface to be 172.16.0.1, the same as my D-Link office router, then I can ping an office computer (172.16.0.xx) from the CNC network (192.168.0.xx), but that's when I run into problems and my office network loses connection to the D-Link's WAN port.

If I set the IP address of the bintec's ethernet interface to be anything else on 172.16.0.xx, I can't ping an office computer (172.16.0.xx) from the CNC network (192.168.0.xx).

I don't think it's right to set the bintec ethernet interface's IP to be the same as my office router, but it's the only way I can see the office computers from the CNC network.  I'm guessing there's a problem with a route missing in the bintec router but I don't know.

Do your Controller PC on CNC network have static or dynamic IP?

The controller PC has a static IP: 192.168.100.101

I disabled DHCP in the laser router.

I disconnected the cable from the ETH port of the laser router and reconnected it to a numbered port (#3).  The other side of this cable is plugged into port 4 of the D-Link router.

I changed the IP of the D-Link to 192.168.100.1 and enabled DHCP from 192.168.100.50 to 192.168.100.100.

I renewed the IPs on the office computers so they were all in the range of 192.168.100.50-100.

Finally I was able to ping everything, from the office to the laser network and back.  The only problem is that the CNC machine wouldn't work properly.  I called tech support for the CNC machine and they said I absolutely must have the office and CNC networks on different subnets.

I really appreciate the help Kechka, it seems like we're close to the solution.  Unfortunately I have to move the office network back to the 172.16.0.xx subnet.